This invention relates to actuating arrangements for aircraft control surfaces.
It is known to provide, in hydraulic actuating arrangements for aircraft control surfaces, a plurality of electrically operated pilot valves which are responsive to nominally identical input signals to control the application of nominally identical servo pressure signals to repective ones of a plurality of intermediate actuators, the intermediate actuators being coupled to a main control valve by means of which an operating pressure signal is applied to an output actuator which is connected, in use, to the aircraft control surface. One of said input signals, the pilot valve responsive to that signal, and an associated intermediate actuator, together comprise a single control channel. Such an actuating arrangement is thus composed of a plurality of control channels and the intermediate actuators are coupled to the main control valve in such a way that any one of said control channels can operate the main control valve, thus providing a level of redundancy which permits the input signals, or the associated devices, in one or more channels to fail.
It is also known to provide hydraulic actuating arrangements having two nominally identical hydraulic pressure supplies, one of these supplies being connected, at any one time, to the pilot valves and to the main control valve. Such arrangements further include a change-over device operable in the event of failure of one of the hydraulic pressure supplies to connect the other of the supplies to the pilot valves and control valve. These latter arrangements have the disadvantage that failure of the changeover device can cause failure of the entire actuating arrangement.